Dogs were experimentally inoculated with Rickettsia rickettsii (canine origin) in order to compare the efficacies of
azithromycin and
trovafloxacin to that of the current
antibiotic standard,
doxycycline, for the treatment of
Rocky Mountain spotted fever. Clinicopathologic parameters, isolation of rickettsiae in tissue culture, and PCR amplification of rickettsial
DNA were used to evaluate the response to
therapy or duration of illness (untreated infection control group) in the four groups. Concentrations of the three
antibiotics in plasma and blood cells were measured by high-performance liquid chromatography.
Doxycycline and
trovafloxacin treatments resulted in more-rapid defervescence, whereas all three
antibiotics caused rapid improvement in attitudinal scores, blood platelet numbers, and the
albumin/total-
protein ratio. Based upon detection of
retinal vascular lesions by
fluorescein angiography,
trovafloxacin and
doxycycline substantially decreased rickettsia-induced
vascular injury to the eye, whereas the number of ocular lesions in the
azithromycin group did not differ from that in the infection control group. As assessed by tissue culture isolation,
doxycycline resulted in the earliest apparent clearance of viable circulating rickettsiae; however, rickettsial
DNA could still be detected in the blood of some dogs from all four groups on day 21 postinfection, despite our inability to isolate viable rickettsiae at that point. As administered in this study,
trovafloxacin was as efficacious as
doxycycline but
azithromycin proved less efficacious, possibly due to the short duration of administration.